forked from luck/tmp_suning_uos_patched
d8a759b570
To improve page allocator's performance for order-0 pages, each CPU has
a Per-CPU-Pageset(PCP) per zone. Whenever an order-0 page is needed,
PCP will be checked first before asking pages from Buddy. When PCP is
used up, a batch of pages will be fetched from Buddy to improve
performance and the size of batch can affect performance.
zone's batch size gets doubled last time by commit ba56e91c9401("mm:
page_alloc: increase size of per-cpu-pages") over ten years ago. Since
then, CPU has envolved a lot and CPU's cache sizes also increased.
Dave Hansen is concerned the current batch size doesn't fit well with
modern hardware and suggested me to do two things: first, use a page
allocator intensive benchmark, e.g. will-it-scale/page_fault1 to find
out how performance changes with different batch sizes on various
machines and then choose a new default batch size; second, see how this
new batch size work with other workloads.
In the first test, we saw performance gains on high-core-count systems
and little to no effect on older systems with more modest core counts.
In this phase's test data, two candidates: 63 and 127 are chosen.
In the second step, ebizzy, oltp, kbuild, pigz, netperf, vm-scalability
and more will-it-scale sub-tests are tested to see how these two
candidates work with these workloads and decides a new default according
to their results.
Most test results are flat. will-it-scale/page_fault2 process mode has
10%-18% performance increase on 4-sockets Skylake and Broadwell.
vm-scalability/lru-file-mmap-read has 17%-47% performance increase for
4-sockets servers while for 2-sockets servers, it caused 3%-8% performance
drop. Further analysis showed that, with a larger pcp->batch and thus
larger pcp->high(the relationship of pcp->high=6 * pcp->batch is
maintained in this patch), zone lock contention shifted to LRU add side
lock contention and that caused performance drop. This performance drop
might be mitigated by others' work on optimizing LRU lock.
Another downside of increasing pcp->batch is, when PCP is used up and need
to fetch a batch of pages from Buddy, since batch is increased, that time
can be longer than before. My understanding is, this doesn't affect
slowpath where direct reclaim and compaction dominates. For fastpath,
throughput is a win(according to will-it-scale/page_fault1) but worst
latency can be larger now.
Overall, I think double the batch size from 31 to 63 is relatively safe
and provide good performance boost for high-core-count systems.
The two phase's test results are listed below(all tests are done with THP
disabled).
Phase one(will-it-scale/page_fault1) test results:
Skylake-EX: increased batch size has a good effect on zone->lock
contention, though LRU contention will rise at the same time and
limited the final performance increase.
batch score change zone_contention lru_contention total_contention
31 15345900 +0.00% 64% 8% 72%
53 17903847 +16.67% 32% 38% 70%
63 17992886 +17.25% 24% 45% 69%
73 18022825 +17.44% 10% 61% 71%
119 18023401 +17.45% 4% 66% 70%
127 18029012 +17.48% 3% 66% 69%
137 18036075 +17.53% 4% 66% 70%
165 18035964 +17.53% 2% 67% 69%
188 18101105 +17.95% 2% 67% 69%
223 18130951 +18.15% 2% 67% 69%
255 18118898 +18.07% 2% 67% 69%
267 18101559 +17.96% 2% 67% 69%
299 18160468 +18.34% 2% 68% 70%
320 18139845 +18.21% 2% 67% 69%
393 18160869 +18.34% 2% 68% 70%
424 18170999 +18.41% 2% 68% 70%
458 18144868 +18.24% 2% 68% 70%
467 18142366 +18.22% 2% 68% 70%
498 18154549 +18.30% 1% 68% 69%
511 18134525 +18.17% 1% 69% 70%
Broadwell-EX: similar pattern as Skylake-EX.
batch score change zone_contention lru_contention total_contention
31 16703983 +0.00% 67% 7% 74%
53 18195393 +8.93% 43% 28% 71%
63 18288885 +9.49% 38% 33% 71%
73 18344329 +9.82% 35% 37% 72%
119 18535529 +10.96% 24% 46% 70%
127 18513596 +10.83% 23% 48% 71%
137 18514327 +10.84% 23% 48% 71%
165 18511840 +10.82% 22% 49% 71%
188 18593478 +11.31% 17% 53% 70%
223 18601667 +11.36% 17% 52% 69%
255 18774825 +12.40% 12% 58% 70%
267 18754781 +12.28% 9% 60% 69%
299 18892265 +13.10% 7% 63% 70%
320 18873812 +12.99% 8% 62% 70%
393 18891174 +13.09% 6% 64% 70%
424 18975108 +13.60% 6% 64% 70%
458 18932364 +13.34% 8% 62% 70%
467 18960891 +13.51% 5% 65% 70%
498 18944526 +13.41% 5% 64% 69%
511 18960839 +13.51% 5% 64% 69%
Skylake-EP: although increased batch reduced zone->lock contention, but
the effect is not as good as EX: zone->lock contention is still as high as
20% with a very high batch value instead of 1% on Skylake-EX or 5% on
Broadwell-EX. Also, total_contention actually decreased with a higher
batch but that doesn't translate to performance increase.
batch score change zone_contention lru_contention total_contention
31 9554867 +0.00% 66% 3% 69%
53 9855486 +3.15% 63% 3% 66%
63 9980145 +4.45% 62% 4% 66%
73 10092774 +5.63% 62% 5% 67%
119 10310061 +7.90% 45% 19% 64%
127 10342019 +8.24% 42% 19% 61%
137 10358182 +8.41% 42% 21% 63%
165 10397060 +8.81% 37% 24% 61%
188 10341808 +8.24% 34% 26% 60%
223 10349135 +8.31% 31% 27% 58%
255 10327189 +8.08% 28% 29% 57%
267 10344204 +8.26% 27% 29% 56%
299 10325043 +8.06% 25% 30% 55%
320 10310325 +7.91% 25% 31% 56%
393 10293274 +7.73% 21% 31% 52%
424 10311099 +7.91% 21% 32% 53%
458 10321375 +8.02% 21% 32% 53%
467 10303881 +7.84% 21% 32% 53%
498 10332462 +8.14% 20% 33% 53%
511 10325016 +8.06% 20% 32% 52%
Broadwell-EP: zone->lock and lru lock had an agreement to make sure
performance doesn't increase and they successfully managed to keep total
contention at 70%.
batch score change zone_contention lru_contention total_contention
31 10121178 +0.00% 19% 50% 69%
53 10142366 +0.21% 6% 63% 69%
63 10117984 -0.03% 11% 58% 69%
73 10123330 +0.02% 7% 63% 70%
119 10108791 -0.12% 2% 67% 69%
127 10166074 +0.44% 3% 66% 69%
137 10141574 +0.20% 3% 66% 69%
165 10154499 +0.33% 2% 68% 70%
188 10124921 +0.04% 2% 67% 69%
223 10137399 +0.16% 2% 67% 69%
255 10143289 +0.22% 0% 68% 68%
267 10123535 +0.02% 1% 68% 69%
299 10140952 +0.20% 0% 68% 68%
320 10163170 +0.41% 0% 68% 68%
393 10000633 -1.19% 0% 69% 69%
424 10087998 -0.33% 0% 69% 69%
458 10187116 +0.65% 0% 69% 69%
467 10146790 +0.25% 0% 69% 69%
498 10197958 +0.76% 0% 69% 69%
511 10152326 +0.31% 0% 69% 69%
Haswell-EP: similar to Broadwell-EP.
batch score change zone_contention lru_contention total_contention
31 10442205 +0.00% 14% 48% 62%
53 10442255 +0.00% 5% 57% 62%
63 10452059 +0.09% 6% 57% 63%
73 10482349 +0.38% 5% 59% 64%
119 10454644 +0.12% 3% 60% 63%
127 10431514 -0.10% 3% 59% 62%
137 10423785 -0.18% 3% 60% 63%
165 10481216 +0.37% 2% 61% 63%
188 10448755 +0.06% 2% 61% 63%
223 10467144 +0.24% 2% 61% 63%
255 10480215 +0.36% 2% 61% 63%
267 10484279 +0.40% 2% 61% 63%
299 10466450 +0.23% 2% 61% 63%
320 10452578 +0.10% 2% 61% 63%
393 10499678 +0.55% 1% 62% 63%
424 10481454 +0.38% 1% 62% 63%
458 10473562 +0.30% 1% 62% 63%
467 10484269 +0.40% 0% 62% 62%
498 10505599 +0.61% 0% 62% 62%
511 10483395 +0.39% 0% 62% 62%
Westmere-EP: contention is pretty small so not interesting. Note too high
a batch value could hurt performance.
batch score change zone_contention lru_contention total_contention
31 4831523 +0.00% 2% 3% 5%
53 4834086 +0.05% 2% 4% 6%
63 4834262 +0.06% 2% 3% 5%
73
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.